solidity

The state or quality of being solid; density; consistency, -- opposed to fluidity; compactness; fullness of matter, -- opposed to openness or hollowness; strength; soundness, -- opposed to weakness or instability; the primary quality or affection of matter by which its particles exclude or resist all others; hardness; massiveness.

That which hinders the approach of two bodies when they are moving one toward another, I call solidity.
Locke.

2.

Moral firmness; soundness; strength; validity; truth; certainty; -- as opposed to weakness or fallaciousness; as, the solidity of arguments or reasoning; the solidity of principles, triuths, or opinions.